Such angle encoders are described in particular in published French patent applications Nos. 2 270 722, 2 304 224 and 2 406 804.
Reference can usefully be made to these documents to fully understand the present invention and the contents of these documents should be considered as being incorporated in the present description by the reference which is hereby made to these documents.
It is recalled that topographical surveys are made by determining the polar coordinates of an observed point in an orthonormalized system having two axes situated in a horizontal plane and a third axis corresponding to the vertical or zenith, the origin of the system being the observation point.
To do this, the equipment used for performing topographical surveys generally comprise a theodolite and a telemeter or other distance measuring apparatus suitable for measuring the distance between the point of observation and the observed point, the theodolite includes a sighting telescope which is pointed towards the observed point and which serves to determine the direction of the sighting axis which passes through the point of observation and the observed point. The theodolite measures both the azimuth of the observed point, i.e. the angle formed by the projection in a horizontal plane of the sighting axis and a horizontal reference axis, e.g. magnetic north, and also the zenithal angle, i.e. the angle formed between the sighting axis and the zenith.
More precisely, the theodolite comprises a horizontal angle encoder adapted to measure the azimuth and a vertical angle encoder adapted to measure the zenithal angle.
These angular encoders are disposed in the alidade, i.e. the moving part of the theolodite which includes the sighting telescope.
To enable a measurement to be made, the alidade is levelled in two directions, for example by means of three screws and under the control of instruments for verifying the degree to which a reference plane of the alidade is horizontal.
The sensitivity of these instruments, generally known as alidade levels is about 10 mgrd or 15.7.times.10.sup.-7 rd, which is sufficient for measuring the azimuth with an accuracy of 0.5 to 2 mgrd.
However, it is clear that the reference accuracy obtained with the alidade leveller is not acceptable for giving the reference zenith with sufficient accuracy to enable measurements performed by the vertical encoder to be as accurate as those performed by the horizontal encoder (i.e. 0.5 to 2 mgrd).
One proposal for achieving this end has been to provide theodolites with a vertical index leveller for correcting errors in theodolite positioning and thus of a vertical index associated with the vertical encoder.
However, this proposal has a major drawback, namely of requiring the vertical index level to be adjusted for each position of the alidade, i.e. at each new angle measurement.
Attempts have been made to remedy this drawback by fitting theodolites with automatic vertical compensators, and this applies both to optical and electronic theodolites.
Such compensation is generally performed by inserting a compensator on the optical path of the light beam used for performing the angle measurement.
Up to the present, two techniques have been used for this purpose in practice.
One of them consists in correcting the reading of the graduated disk integrated in the encoder by means of a prism suspended from a cross-wire pendulum.
The other consists in correcting the reading of the graduated disk by refracting the light beam through a liquid of constant refractive index or by means of data related to the reflection of radiation at the surface of the liquid, e.g. mercury, as is described in U.S. Pat. No. 4,108,539.
The design of these compensating devices, i.e. for compensating the reference zenith, requires the vertical angle encoder to be disposed coaxially with the axis of rotation of the sighting telescope.
This is naturally an encumbrance when it comes to placing the various components required in a compact theodolite.
Further, the maximum range of compensation is limited to about .+-.0.1 grd to .+-.0.5 grd.
Preferred embodiments of the present invention provide a novel angle encoding device which provides direct measurements of the zenithal angle from 0 to 200 grd with good accuracy.